1. Field of the Invention
The invention relates to xenograft heart valves and more particularly relates to a method of fixing a xenograft mitral heart valve for use in humans.
2. Description of Related Art
Heart valves from human cadavers, so called "homograft" valves, have been implanted in living human recipients for at least thirty years. Evolution of the Homograft Valve, Donald N. Ross, Special Presentation, Annals of Thoracic Surgery 59:565-7 (1995). A primary problem with homograft valves is their availability. By contrast, heart valves from animals, so called "xenograft" valves, particularly from pigs, cows and sheep, are abundant. Porcine xenograft heart valves have been collected, treated and implanted in humans to replace damaged or defective human heart valves since at least the early 1970s.
Porcine aortic heart valves have been found to be particularly good replacement heart valves for humans. One reason for this is that the porcine aortic heart valve has many similarities in both size and structure to the aortic valve in human hearts.
With respect to the mitral valve, a porcine mitral valve of a certain annular size generally has chordae tendineae slightly shorter than the chordae tendineae of the human mitral valve it replaces with a similar annular size. This problem has been solved, as disclosed in my co-pending U.S. patent application Ser. No. 08/566,229 titled "Physiologic Mitral Valve Bioprosthesis" filed Dec. 1, 1995, by fashioning a porcine mitral xenograft valve that is suitable for human implant.
Throughout this disclosure, the term "subvalvular" means the part of the heart below the annulus of the mitral valve including the chordae tendineae and papillary heads and the term "annulus" means the part of the heart where the leaflets are inserted into the endocardium or inner wall of the heart.
Many factors can be used to choose the porcine mitral valve to be implanted in the human. Many candidate porcine valves are rejected because their shape, cosmetic appearance, or chordal distribution and geometry are not desirable. If any of these characteristics is altered during the fixation process, the once desirable valve would become unusable. The method of fixing a mitral valve such that its annulus and subvalvular geometric structure is not altered during the fixation process is a problem in need of a solution.
A method for fixing a porcine xenograft mitral valve has been tried by M. P. Vrandecic et. al. Anatomically Complete Heterograft Mitral Valve Substitute: Surgical Technique and Immediate Results, Journal of Heart Valve Disease 1992;1:254-259. Vrandecic's method uses a surgical technique to harvest the mitral valve immediately after slaughter in order to preserve the integrity of the mitral valve. The valve is then fixed using zero pressure fixation. However, in removing the valve from the heart prior to fixation, the valve may suffer geometric distortion because of the lack of the left ventricular tethering function which serves to maintain mitral valve geometry. The result may be a valve that fatigues prematurely due to unnatural stress distributions in the leaflets, annulus or the subvalvular apparatus. Therefore, the problem still exits as to how to fix a xenograft mitral valve in its natural configuration.